docs: def _csv_header(self):
          """
          Extract the expected CSV header from the exposure metadata
          """
          fields = ['id', 'number', 'taxonomy', 'lon', 'lat']
          for name in self.cost_types['name']:
              fields.append(name)
          if 'per_area' in self.cost_types['type']:
              fields.append('area')
          if self.occupancy_periods:
              fields.extend(self.occupancy_periods.split())
          fields.extend(self.tagcol.tagnames)
          return set(fields)